A form factor specifies the physical dimensions of a system. Basically it is the motherboard form factor that defines the overall size of a system. There are dozens of standardized form factors. Among them we will be concerned with the following three most popular form factors.

Mini-ITX: 170mm x 170mm (6.7″ x 6.7″)

MicroATX: 244mm x 244mm (9.6″ x 9.6″)

ATX: 305mm x 244mm (12″ x 9.6″)

Because of the size, Mini-ITX provides the least expandability (0 or 1 expansion slot), usually 2 memory slots and CPU support is often limited by the cooling performance of a small Mini-ITX system. MicroATX supports up to 4 expansion slots, while ATX supports up to 7 expansion slots. Usually a Mini-ITX/microATX motherboard supports an integrated graphics so that you may not need a discrete graphic card. An ATX case can usually hold more storage drives than an microATX case, and a microATX case can hold more storage drives than an Mini-ITX case.

Here is a physical comparison of actual Mini-ITX motherboard/case, microATX motherboard/case and ATX motherboard/case, along with an AV receiver.

You may wonder why the width of the microATX case is almost the same as that of the ATX case. The reason is simple: the PSU is usually laid flat in a microATX case, while it is laid vertically in an ATX case. As a consequence, a microATX case is usually shorter in height than an ATX case.

Streamed media such as TV and radio (terrestrial, satellite, cable, Internet).

Media files stored locally.

Creating media files from various sources, non-streaming or streaming (usually called "ripping" or "recording"), and storing them for later use.

Editing, including re-encoding, media files.

Hardware components that are important for each task is:

Playing back video: This includes decoding and various post-processing (deinterlacing, rescaling etc.). GPU is the most important for this task (unless you resort to a software playback solution such as ffdshow). A couple of GPUs integrated in motherboard are good. If you want to get the best picture quality, a good mid-range discrete GPU is recommended, however. A high-end card is good for better gaming experience of course, but it rarely improves video playback performance.

Ripping: The speed of ripping DVD/BD discs is often limited by the reading speed of the optical disc drive used.

Recording: HDTV contents are already encoded in either MPEG-2 or H.264. So this is very easy for any system.

Editing and re-encoding video (except for simple cut and join): This is one of the most CPU-intensive tasks. A good quad-core (or more) processor is recommended. A trend is that GPU (stream processors) offloads CPU, and several video editing applications already support it (keywords: GPGPU, OpenCL, Microsoft DirectCompute, NVIDIA CUDA, AMD APP).

CPU, chipset (in motherboard; controlling various I/O devices and connecting them to CPU/memory) and GPU are the three main hardware components of a system. Intel and AMD are the main suppliers of CPU for PC. Intel and AMD are producing chipsets for its own CPUs. Intel (integrated GPU only), AMD and NVIDIA are the top three GPU manufacturers.

Intel produces chipsets for its own chips and AMD for its own chips. ASUS, ASRock, GIGABYTE and MSI are the top four motherboard manufacturers and their motherboards are in general very reliable. The performance of a motherboard is mostly determined by the chipset and there is little difference between the manufacturers. So the main selection factor is the features of the motherboard, e.g. the number of PCI Express slots, USB 3.0, IEEE 1394.

DDR3 SDRAM is the mainstream memory standard and you should choose it unless you already have DDR2 memory modules.

Capacity, frequency, timings, voltage

A pair of 2GB memory modules, 4GB in total, is standard right now. Currently both Intel and AMD desktop processors support up to DDR3-1333. Considering slight overclocking capability, DDR3-1600 is a good choice. CAS latency (CL) and timings are important for memory-intensive applications, in particular games. However these have little effect on the majority of HTPC-related tasks. So just ignore them. Even in games, memory affects the performance the least among CPU, memory and GPU. You'd better spend money on better CPU/GPU instead of spending money on expensive "performance" memory modules. The standard operating voltage of DDR3 SDRAM is 1.5V. Some memory modules require higher voltage than that for better stability. Adjust the memory voltage in BIOS according to the specifications of your memory modules.

Brand

Basically the brand does not matter in performance as the standards are established by JEDEC rigorously. It's not like Intel vs. AMD in CPU. Reliability and overclockability may vary from brand to brand, however.

With the advent of Blu-ray Disc (and HD DVD), HDMI became the standard specifications for transmitting video and audio signals from a player/PC to an AV receiver/display. Right now there are basically three HDMI solutions in PC:

AMD Radeon HD 5xxx graphics cards; to be replaced by HD 6xxx Series in Q3 2010 and Q1 2011.

In case you have no idea what to choose (and no time to dig), here are my pick. The system provides the best video/audio playback performance, as well as reasonably good performance/low power consumption in various CPU intensive tasks. If you have (or will have) a HDMI 1.4a 3D HDTV or projector and are interested in 3D videos, choose GeForce GT 430.

A MCE remote is a (usually IR) remote control and a receiver to be attached to the HTPC (internally or externally via USB) that controls Windows Media Center and other front ends/media players at a distant place. Some HTPC cases and TV tuner cards come with a MCE remote. If not, you can buy one.

These cards enable any PC running Windows 7 Media Center on your local network to watch or record up to four (Ceton; the max number of CableCARD tuners Windows 7 allows)/three (HDHomeRun) live cable channels at once, including premium channels. You just need:

Digital cable subscription from a US cable provider

Multi-Stream CableCARD (M-Card) available from your cable provider

A limitation on recordings is:

A content marked as Copy Freely has no DRM in your recording. It is basically the same as clear QAM contents once decrypted by the M-Card.

A content marked as Copy Once can be watched only on the PC where it was recorded and Media Center Extenders like the Xbox 360.

It is up to each cable provider which content is marked as Copy Freely/Copy Once.

This device captures HD video contents, encrypted or unencrypted, via component video (i.e. analog) from a cable or satellite TV set top box in H.264 video with DD or AAC audio. Recordings are naturally DRM-free. You can watch/record one channel at a time with a HD PVR unit and a STB. You will need multiple HD PVR units and multiple STBs to watch/record multiple channels simultaneously.

If you are going to use an AV receiver, you are unlikely to use a discrete sound card because necessary hardware for HD digital audio is provided by either the motherboard's onboard audio codec or the HDMI on a graphics card (read Introduction: Component Selection: Graphics and Sound Devices). Here are some exceptions.

Some people prefer an analog sound card to (the DAC and preamplifier processor part of) an AV receiver for several reasons. If you fall into this category, here is a list of a couple of good sound cards:

Note that if you use a commercial BD software player, HD audio will be downsampled with these cards. Ripping BD movies into another format such as MKV and playing them with appropriate audio decoders is a way around. For example, please read this post.

Microsoft Windows is the dominating OS in HTPC for good reasons. The latest Windows is recommended.

Windows 7, Home Premium or higher, 32-bit or 64-bit, Retail or OEM or TechNet/MSDN Subscription.

Remarks

Windows 7 Editions: Home Premium is enough for normal HTPC tasks. 7 Home Premium supports full-system backup and restore unlike Vista Home Premium. You can find comparison of Windows 7 editions in this Microsoft web page and Wikipedia.

32 bit vs. 64 bit: Advantages of Windows 64 bit are:

It supports more than 4GB memory; however this is not important because normal HTPC tasks won't get benefit from more than 4GB memory.

If you are going to boot from a HDD over 2.199TB capacity, you will need a motherboard with UEFI (Unified Extensible Firmware Interface) and Windows 7/Vista 64-bit. (The 32-bit OS supports a data storage HDD over 2.199TB just fine.)

Disadvantages of Windows 64 bit are:

Windows Media Center of Windows 64-bit is a 64-bit application and hence you can't use a couple of useful 32-bit DirectShow filters with its internal video player (e.g. madFlac Decoder, ReClock, ArcSoft Audio Decoder, ArcSoft Video Decoder, CyberLink Video Decoder). This is not a fatal disadvantage however even if you use Windows Media Center as a front end; you can always use an external media player such as Media Player Classic HomeCinema.

So which one to choose? Personally I recommend Windows 32-bit. All the video playback applications (except for Windows Media Center in Windows 7 64-bit) are still 32-bit and there is no point of using the 64-bit OS. The 64-bit version simply creates complications in configuring the system for 32-bit players with zero performance benefit. By the time media players and video/audio codecs, in particular commercial Blu-ray players, are developed in 64-bit, Windows 7 will have been outdated anyway.

TechNet Subscription: You may want to subscribe TechNet Standard, $199 per year, or TechNet Professional, $349 per year, if you want to install Windows (including Windows Home Server), Microsoft Office, and/or other Microsoft applications on several (non-productive) machines. The license is valid indefinitely. You can activate a product with the supplied product key even after your subscription expires, up to 10 times per product key. You can obtain 2 retail keys for each product in Standard and 5 retail keys for each product in Professional, so that 2 x 10 = 20 times or 5 x 10 = 50 timesactivation is allowed in several machines for each product. Windows 7 Home Premium, Professional and Ultimate are counted as different products, but the 32-bit version and the 64-bit version of each edition are counted as the same product. You can't download products or obtain product keys after the expiration of your subscription, of course.

Mini-ITX form factor has become popular because of its small footprint. However there are a couple of limitations. It supports at most one expansion slot. So if you decide to add a discrete graphics card, for example, you can't use an internal TV tuner card. A Mini-ITX case is usually very small, hence there is often a space limitation for the CPU cooler, storage drives and PSU. In many cases you can use only a slim-type optical drive and/or a 2.5″ HDD, that are often expensive. If you agree with these limitations, a Mini-ITX system can be not only a good HTPC but also a gaming machine or a video encoding machine by placing a powerful discrete graphics card and/or CPU.

Optical or Coax: This indicates the existence of an S/PDIF connector (optical or coaxial) on the rear panel of the motherboard.

DDL or DTSI: DDL is Dolby Digital Live, DTSI is DTS Interactive. These technologies encode multichannel LPCM sent to the audio codec of the motherboard into Dolby Digital or DTS in real time. A convenient tool to get surround sounds from games over S/PDIF.

Analog Audio (MB)

Channel Count: The supported number of channels of analog audio from the audio codec of the motherboard.

PAP: Support for Protected Audio Path. If this exists, you may be able to get full quality lossless analog audio from Blu-ray Disc movies when you use a (not necessarily every) commercial Blu-ray Disc software player.

Other Features (MB)

SATA 2.0: The number of SATA 2.0 (3Gbps) internal connectors.

eSATA 2.0: The number of eSATA 3Gbps connectors on the real panel.

USB 3.0: The number of USB 3.0 connectors on the rear panel if exists.

Power per Frame: The AC power draw of the total system to encode a frame in the above benchmark. The value depends on PSU's efficiency, which depends on various factors. So take it as a rough estimate.

Power Consumption (DC): The DC power draw of the total system excluding PSU. The AC power draw from the wall is (DC power draw)/(Efficiency of the PSU).

Summary Instead of a pricier AMD chipset, this system uses a 4-year-old NVIDIA chipset, with a better discrete graphics card. Video playback performance is better than the above system and HD audio bitstreaming is supported.

This system is specifically intended for 3D videos in a HDMI 1.4a HDTV/projector. The graphics card can decode Blu-ray 3D codec and output in the Frame Packing format. CPU is powerful enough for 2D to 3D conversion.

Optical or Coax: This indicates the existence of an S/PDIF connector (optical or coaxial) on the rear panel of the motherboard.

DDL or DTSI: DDL is Dolby Digital Live, DTSI is DTS Interactive. These technologies encode multichannel LPCM sent to the audio codec of the motherboard into Dolby Digital or DTS in real time. A convenient tool to get surround sounds from games over S/PDIF.

Analog Audio (MB)

Channel Count: The supported number of channels of analog audio from the audio codec of the motherboard.

PAP: Support for Protected Audio Path. If this exists, you may be able to get full quality lossless analog audio from Blu-ray Disc movies when you use a (not necessarily every) commercial Blu-ray Disc software player.

Other Features (MB)

SATA 2.0: The number of SATA 2.0 (3Gbps) internal connectors.

eSATA 2.0: The number of eSATA 3Gbps connectors on the real panel.

USB 3.0: The number of USB 3.0 connectors on the rear panel if exists.

Power per Frame: The AC power draw of the total system to encode a frame in the above benchmark. The value depends on PSU's efficiency, which depends on various factors. So take it as a rough estimate.

Power Consumption (DC): The DC power draw of the total system excluding PSU. The AC power draw from the wall is (DC power draw)/(Efficiency of the PSU).

I will use APEX DM-387″ the budget systems, nMEDIAPC HTPC 5000B in the low-end systems, Antec Fusion Remote Black in the mid-range systems, Lian Li PC-C50 in the high-end systems and LUXA2 LM200 Touch in the premium systems.

Here are systems at as low cost as possible, but with good performance of HD and SD video playback. You can always add a better discrete graphics card later such as PowerColor AX5450 512MK3-SH Radeon HD 5450 GDDR3 512MB, $40.

The system uses the older chipset G41/ICH7 with integrated graphics. The integrated graphics GMA X4500 does not support hardware decode acceleration for HD video codecs, hence you may see high CPU usage, 60%--70%. Otherwise it is the same as GMA X4500 HD (the GPU integrated in G45) in video and audio performance. In particular video post-processing is very good and HDMI audio supports multichannel LPCM.

The Intel Core i3 system in the previous mid-range system section is now in the low-end system section. The integrated GPU is overall very good with a few issues such as non-support for 23.976Hz refresh rate and not so good support by free and open-source video codecs.

Summary Instead of a pricey Core i3 processor and a H55 motherboard, you can use a much cheaper Intel Celeron processor and a motherboard with older chipsets, but add a better discrete graphics card for overall better video playback performance at a lower cost.

Summary AMD iGPU is somewhat limited in today's standard. If you want to add a discrete graphics card, then you could choose a cheaper motherboard whose onboard video is inferior but you won't use anyway.

The ATX form factor supports up to seven expansion slots (vs. four in microATX) and a larger ATX case can usually hold more storage drives. Moreover motherboard manufacturers tend to implement better CPU power circuitry and cooling solution for MOSFET/chipset in ATX motherboards than microATX motherboards.

Optical or Coax: This indicates the existence of an S/PDIF connector (optical or coaxial) on the rear panel of the motherboard.

DDL or DTSI: DDL is Dolby Digital Live, DTSI is DTS Interactive. These technologies encode multichannel LPCM sent to the audio codec of the motherboard into Dolby Digital or DTS in real time. A convenient tool to get surround sounds from games over S/PDIF.

Analog Audio (MB)

Channel Count: The supported number of channels of analog audio from the audio codec of the motherboard.

PAP: Support for Protected Audio Path. If this exists, you may be able to get full quality lossless analog audio from Blu-ray Disc movies when you use a (not necessarily every) commercial Blu-ray Disc software player.

Other Features (MB)

SATA 2.0: The number of SATA 2.0 (3Gbps) internal connectors.

eSATA 2.0: The number of eSATA 3Gbps connectors on the real panel.

USB 3.0: The number of USB 3.0 connectors on the rear panel if exists.

Power per Frame: The AC power draw of the total system to encode a frame in the above benchmark. The value depends on PSU's efficiency, which depends on various factors. So take it as a rough estimate.

Power Consumption (DC): The DC power draw of the total system excluding PSU. The AC power draw from the wall is (DC power draw)/(Efficiency of the PSU).

There are many good ATX cases. Here is a list of well-built ATX cases. All SilverStone cases except for LC16M/LC20(M)/GD01(MX), Antec Fusion Remote Max, and Thermaltake DH103 and DH104 can hold a graphics card of any length, possibly by removing a HDD cage.

The Intel Core i3 system in the previous mid-range system section is now in the low-end system section. The integrated GPU is overall very good with a few issues such as non-support for 23.976Hz refresh rate and not so good support by free and open-source video codecs.

Summary Instead of a pricey Core i3 processor and a H55 motherboard, you can use a much cheaper Intel Celeron processor and a motherboard with older chipsets, but add a better discrete graphics card for overall better video playback performance at a lower cost.

Summary AMD iGPU is somewhat limited in today's standard. If you want to add a discrete graphics card, then you could choose a cheaper motherboard whose onboard video is inferior but you won't use anyway.

If you need more storage space than the ones provided by the motherboard and the case, a quick solution is attach an external HDD enclosure. If you need lots of drives, you may want to consider a dedicated file server (see Media Storage Server). If you need higher I/O throughput, e.g. for video editing, there are many SATA RAID controller cards. Here are a couple of examples.

The next class of enclosures uses JMicron JMB393/JMB394 5-port SATA 2.0 port multiplier with RAID function, along with a USB 3.0 to SATA 2.0 bridge. Some of them come with a (SATA 3.0 PCIe 2.0 x1) host adapter, but this is unnecessary if your motherboard has an eSATA 6Gb/s connector or a USB 3.0 connector.

JMicron JMB393/394 supports PM or non-PM aware host. Thus you can connect it to any eSATA port of your system in hardware-accelerated RAID mode. Sans Digital claims that when connected to a SATA 6Gb/s port, you can expect in RAID 5

Workstation here is a PC intended for various HTPC-related tasks other than video/audio playback. The CPU and the motherboard chosen here are good enough for the most demanding tasks and future upgrades (4 cores/8 threads, supporting PCI Express 2.0 x16, x16 / x16, x8, x8 / x8, x8, x8, x8 links). The selection of other components depends on the tasks you are going to do. The system below is a consumer-grade high-performance video editing machine.

Windows Vista or 7, 32 bit or 64 bit, with or without FlexRAID and/or FlexRAID-View

Windows Home Server (WHS), with or without FlexRAID (in place of Folder Duplication)

unRAID (a Linux variant with RAID 4 capability)

You can also use Linux with software RAID, OpenFiler etc. as long as your hardware components are supported.

Case

NORCO RPC-4220/RPC-4224 is the best case for a media storage server. It support 20/24 HDDs in hot-swap bays and has a backplane with five SFF-8087 mini-SAS connectors for better cabling at a relatively cheap price. If you need more storage space, build another server of the same type and store them in a rack such as iStarUSA WO22AB 22U WO Open Frame Rack, ~$300.

PSU

Each hard disk drive consumes as low as 5W at idle, but as much as 30W at start-up, depending on the model. So we may need a powerful PSU to start up all the drives at a time. A typical power consumption of a 20-HDD server is

~500W at start up

~200W at seek

~150W at idle

CPU

A dual-core processor is enough because archiving/streaming/recording is not so CPU-intensive.

Memory

2GB is enough for a similar reason.

Motherboard

Basic requirements are

Onboard graphics

Six or more onboard SATA ports for additional storage HDDs and the HDD for OS

You will need to back up important, irreplaceable data (e.g. personal documents and family photos), perhaps off-site. Building a dedicated backup system is one method (you can use similar hardware components here). A caution to those who consider RAID5: RAID 5 is not a backup strategy, it's about uptime - if a drive fails, you can swap it for a new one to rebuild with no service interruption. You may lose the entire date in the array instantly however if multiple drives fail or if a hardware issue kills the array. FlexRAID and unRAID are better in this point: you can still get data from each non-failed drive.

Motherboard: ASUS M4A78LT-M AM3 AMD 760G/SB710 chipsets microATX, $68. The LE version (ASUS M4A78LT-M LE) is reported to have LAN issues under unRAID. The non-LE version has a Realtek LAN chip, while the LE version has an Atheros LAN chip.